1. Field of the Invention
The following generally relates to a digital receiver and, more particularly, to a method and apparatus for providing an energy based signal tracking loop within a digital receiver.
2. Description of the Related Art
A global navigation satellite system (GNSS) typically includes multiple orbiting satellites and at least one receiver (GNSS receiver). Examples of GNSS include the global positioning system (GPS), the Galileo positioning system (GALILEO), the Global'naya Navigatsionnaya Sputnikovaya Sistema (GLONASS), and the like.
In operation, each of the satellites broadcast a radio signal (GNSS signal) upon which is modulated certain information. The GNSS receiver captures the broadcast satellite signal, extracts the information modulated upon the signal, and computes an estimate of the position of the GNSS receiver using the information. More specifically, the receiver position is determined by computing, for each satellite in view of the GNSS receiver, a time delay between the time of the transmission from the satellite and a time of reception of the satellite signal at the GNSS receiver. The time delay multiplied by the speed of light provides a distance (a pseudorange) from the GNSS receiver to the satellite. Using pseudoranges to a number of satellites, the receiver computes the position.
To enable the receiver to extract information from the GNSS signals, the carrier frequency and the code delay (which is indicative of the pseudorange) must be determined and tracked. The GNSS signal is modulated with a pseudorandom (PN) code that is correlated with a PN code in the receiver. The correlation process produces an estimate of frequency and code delay.
These estimates vary over time due to motion of the receiver relative to the satellites, satellite motion through space, clock inaccuracies, and the like. As such, time and frequency tracking loops are used. These loops utilize a discriminator that derives a time and/or frequency error and applies the error to the frequency or timing loop to compensate for the error. Such loops (e.g., phase or frequency lock loops) are well known and are widely used in GNSS receivers.
One issue that arises with the use of conventional phase or frequency tracking loops is that they rely upon a discriminator, that uses a nonlinearity. Such discriminators are not sensitive to low-signal strength signals. Consequently, at low-signal strength, conventional signal tracking loops fail.
Therefore, there is a need in the art for a tracking loop that tracks delay and frequency of low-signal strength signals.